1978 cb125 single - just making it go

against88

New Member
hello.
my first post. many of my searches brought me here so i figured i'd start a thread.
(unfortunately i just have this one photo to offer)
https://flic.kr/p/nWkyu9


i bought a 78 cb125 single to fix up for my girlfriend. i wanted a project and figured a small bike would be cheaper and easier to work on. i am quasi intelligent, but an absolute novice when it comes to mechanics, so this is really a learning process for me.

this post is really long so here is the abridged version:
-kill switch doesn't work. will open that eventually
-new battery was dead when i got back after 3 weeks away w/ ignition off. any reasons this might happen? will replace rectifier w/ silicon and clean any electrical/ground connections i can access.
-front disk brake. all parts seem present. how do you adjust? (only manual i found is "shop manual honda 100/125" including 125s. only shows front drum brakes)


it came to me as not starting, but having been working within the past few months. within 6-8 months it had 3 owners, and i had 2 receipts that were pretty vague. it seems like someone would buy it, it stopped working, shop got it working, a month of riding, stopped working, get frustrated, sell.

it seemed to have compression, and the cylinder smelled of fuel, so i checked for spark (grounding the plug) and got nothing. checked the battery, dead and bone dry (also held on with zip ties). picked up a new battery and spark plug, checked for spark, got it! with a few kicks it fired up and idled well. great success. it is a little finicky getting into 1st/neutral, but power seems to be there and otherwise shifts well. clutch is a little lose feeling, but not bad.

i wanted to check/clean the oil filters, as i read that they are very under serviced on these bikes, but i didn't have time/patience to crack the right side engine cover. with a little more investment in the bike, i will, but for now, i just drained and replaced the oil. old oil looked decent, so that's sort of promising.

kill switch doesn't work at all. will have to look into that.

foot pegs/kick stand is missing 2 bolts. heading to hardware store to grab those.

i just did a cross country road trip on a KLR from FL back to LA, and when i returned the battery was dead. ignition was in the off position, and the battery was very new (rode the bike on/off for a few days/week before i left), any ideas on what would be slowly draining a fresh battery? i have plans to pick up a radio shack silicon rectifier as i have read that it's worth replacing whether you go 12v or stay 6v, basically anything better than the original.

headlight has a very DIY on/off/on switch, and its very dim when switched on, blinkers don't work, but brake light seems reasonably strong. i assume there are some corroded connections, regardless of whether there are other issues (i assume there are) so cleaning some contacts is definitely in order. i'd love to clean up and re-wire it as i suspect there are some weird random psuedo-fixes, but that will wait.

front mechanical disk brake. didn't do much but slow the bike a little little bit. i understand this is a weak point as people tend to swap to drum brakes, so i don't expect much. i opened the brake up today and black water dripped out. poured out a small pool of it from the brake assembly cap. everything looks like it is supposed to be there, the idea seems simple, pull cable, cable pulls pads, spring pushes back to zero. wiped away some schmutz and yellow crust, and put it back together. with the three housing bolts tightened down, the brake lever now had zero travel, and the brake was locked up. loosened the bolts and got a little movement in the tire and lever. loosened the bolts more (now barely finger tight) and the lever functions, but the brake is weak (possibly a bit better than it started). so it seems promising that this thing has the potential to stop the tire, but how should this thing be adjusted??

oh, i also tried to adjust the valves, but i couldn't get the dynamo cover off to check the timing. friend has an impact driver, but not sure how to use it... does it turn/ratchet when hit, or do you have to apply rotational force while you hit it?

thanks, sorry for the verbosity!
-will
 
Very cool. More photos please!

This section is fine for what you are doing.

Welcome aboard,
Damian.
 
you're right about the mech disc brake...they were crap from new. if i got it right then you were undoing the actual caliper mounting bolts to get it to unlock ? not a good idea. the bolts will fall off , closely followed by the caliper itself. the caliper should be firmly bolted to the fork leg at recommended torque settings (see relevant manual/check internet or just do up fuckin' tight) and all adjustment should be done via the cable. this should at the very least have an adjuster at the lever end (same as the one on the clutch lever) it sounds as though your cable is too tight if its locking the caliper onto the disc, see if you can loosen the adjuster at the lever (screwing it back in towards the lever), also lubricating the cable and all moving points will help things along (obviously keeping all slippery substances away from the faces of disc pads or the disc itself)

also with the impact driver, turn the collar to L (stands for Left or Undo) place driver bit into screw head and turn the body of the driver to the left whilst pushing it down, this takes up all the slack and will allow the force from your favourite hitting stick to go direct to the head of the screw rather than taking up that slack (for impact driving i use my 'special' hammer, a 5lb lump hammer, a normal hammer generally doesn't have the 'oomph' for impact driving)
while you've got all those original POS screws out, take them down to your local bolt suppliers and have them match them up to shiny new stainless steel 'hex' heads (also called allen head bolts), they won't rust or chew out the heads and if you use a tiny bit of copper or aluminium grease on the threads when you put them back in, they won't seize into the cases either. do the same with all case bolts either all at once or as you find yourself taking them out of various cases for repairs or maintenance on the rest of the motor
 
re: brakes

when i got the bike, the front brake lever squeezed, but had almost no impact. basically none. so i opened what i thought was the brake mechanism housing. (not sure if this whole assembly is considered the caliper). 3 bolts as seen in this photo:



everything looks to be present, and sort of functioning. (with bolts removed, the brake lever pulls cable, against spring, and seems to compress a few greasy washer type discs, as well as the white plastic end piece. i notice that when a bit of pressure is put on the white plastic piece, its teeth/gears pull on a thin metal washer with angled arms, then upon release it slides over these teeth, a ratcheting motion.)

the problem is that with the casing off and no pressure on the white plastic end (and the whole assembly) there's healthy brake lever movement, but no braking action. when the casing is closed and three bolts are tightened any reasonable amount, the brake lever is fixed, and the brake is locked.

i tried adjusting the disc side of the cable with these two hex bolts/nuts, but it didn't seem to make much difference in either unlocking the brake, nor inducing braking action. two hex bolts in question:



unfortunately, the lever end of the brake cable has no adjustable parts.
also, my manual doesn't include the disc brake so i can't reference that.

seems like its all there, i just need a better understanding of why closing this casing would lock the brake, and why it wasn't doing that originally.

thanks for any input!
 
methinks i see the problem, the big black thingy is indeed the caliper and those 3 big bolts should be done up quite tight indeed (in the absence of a torque wrench or the actual torque settings we'll settle for one and a half grunts....). i'm assuming you have already lubed the cable and pivot points at both lever and caliper ends

it will help if you can set the bike up with the front wheel off the ground to do the adjustment

those two hex heads are the adjustment system, loosen the lower one (called the lock nut) off and wind it all the way up the threaded bolt (the adjuster), then wind the adjuster back into the caliper all the way. this should give you back movement at the lever but probably too much and also no actual braking

now wind the adjuster back out of the caliper and keep spinning the wheel every couple of turns of the adjuster, you'll feel the wheel starting to drag at some point, at that point screw the adjuster back into the caliper a turn or so then wind the lock nut back down the adjuster til it hits the caliper, give it a light nip up with a spanner (i think you might know it as a wrench)

if all has gone well you should still be able to spin the wheel, have a little bit of slack at the lever and when you pull the lever it should stop the wheel spinning. now try it with the bike back on the ground while wheeling it back and forward in the yard/garage/driveway with you sitting on it.

if all is well its time for a test ride, take it easy and slow and see how it feels, it may need a touch more or less adjustment, take the spanners with you

after typing all that lot i found these two youtubevids which are about a CB200 but it appears to be exactly the same caliper as you have so i hope they help

http://www.youtube.com/watch?v=dKlIVUi5Vxo

http://www.youtube.com/watch?v=cWF_KDpVRCE
 
haven't been back to the bike, but i'm watching videos and getting a better grasp of what i'm looking at.
(video is much better for me than an exploded diagram!)

quick question, is there a temporary grease alternative that i might have on hand such as chain lube (spray) or motor oil?

and i'm sure i'll forget it lots more before my work is done, but thanks for walking a newb through this!
 
very related question:

any consensus on a few drop in replacements for wheels?
i'm assuming any cb125 with a front drum would be a no brainer, but anything else to keep an eye out for?
 
against88 said:
quick question, is there a temporary grease alternative that i might have on hand such as chain lube (spray) or motor oil?

hiya, problem with using thin oils such as those is that under braking conditions, you get heat generated and that heat will transfer through all the components in the caliper, this in turn will heat up whatever lube you've used and thin it out, it can then run down/round/out from where it is and eventually end up on the disc itself. when this happens your braking capacity will go from crap to non-existent.

you really need to get some copper or aluminium based grease (i got mine from the local bolt shop)
these greases are much more tolerant of high temps and are also a good idea to use on the mounting bolts and adjusters as they have anti-seize properties, making it much easier to strip/adjust the components in future

on a side note people also use this sort of grease when putting steel ot stainless steel bolts into aluminium engine cases as it stops them seizing into the cases

this is a link to local (to me) suppliers of copper grease thru ebay but if you do the same search you'll find ones local to you, and as you'll see its not that expensive

http://www.ebay.com.au/sch/i.html?_trksid=p2050601.m570.l2632.R2.TR2.TRC1.A0.H0.Xcopper+grease&_nkw=copper+grease&_sacat=6030&_from=R40

sorry, got no idea about wheels for little bikes, but i'm sure someone on here will be able to help, maybe start a new thread with the specific request in the title (maybe something like CB125 disc to drum swap - what wheels?) and someone will pop up to help
 
didn't get a chance to mess with the front brake, but i did replace the rectifier. (whether or not it was the problem)

charged the battery to 6.1v
rewired the rectifier using the wire color guide from a cb/cl 450(?) thread
crossed fingers
started bike on third kick
rode around!
got over zealous, fishtailed a bit when i remembered that going is good, stopping, not so much.
measured voltage @ battery terminals of 7.3-8v depending on if headlight or signals were on or not. figured that was pretty good.
bike off. measured 6.3v
not great, but seems to indicate that my 2 miles around the block means (i think) that the system is charging the battery.

will update next time i'm at the garage if the battery held its charge or not.

plus the whole front brake thing.

also, its a bit hard to find 1st gear at times. it likes to click down from 2nd to 2nd.

happy for today.
 
ok, its been a week and the bike fired up first kick.
pretty excited that the battery held its charge so there doesn't seem to be any slow drain issue.

opened up the front brake after reviewing the videos spotty posted.
i think last time i opened them up, i may have clicked the racheting mechanism over a few times, thereby pushing the center (advancing?) screw out a bit, making the brake think that the brake pad was further worn than it was. so when i tightened down the housing, that was enough pressure to apply braking force to the disc. cleaned up the system a bit, backed out the center screw, added some aluminum based grease, and closed it back up. initially i got no braking force, but as i adjusted the adjustment nut on the brake cable, i got my braking back. its certainly not great (could've probably went another turn or two), but it'll slow the bike down. the lever doesn't really spring back, i assume that's from the spring being fatigued.

bike started, and now stops a bit better. another good day.

idle acted funny at one point, dropping very low, but a blip of the throttle remedied that.

next on the list is more brake tweaking, cleaning (just a wipe down/spray down), and taking a look at the funky wiring (there is a blade style automotive fuse hanging from a random splice near battery world... interesting.

thanks!
 
here is a random picture that makes the bike look a little cleaner than it is. missing the side cover. and the foot pegs seem to be angled upward more than they should be.
14507632359_1893c20b50.jpg





this is part of the wiring mess i will deal with eventually.
14691103991_6b0b5738d8.jpg
 
glad i was able to help with the brakes, you're on your own with the electrickery though....

seriously though, there'll be someone hereabouts with the knowledge to get you started, just try and find a wiring diagram before you start asking, see what doesn't look right on the bike and compare it with what should be there according to the diagram - if you can't work it out, then ask questions
 
Cool bike - can you hear it run with the giant stock muffler? Might sound nice with a shorty reverse megaphone...
 
Sam,
Thanks!
It's a little rough around the edges, but so far it's been starting/running reliably!
And regarding the muffler, I don't think I've seen one exactly like this, and it certainly looks nerdy, but its not that quiet. Sounds decent, in fact it's louder than my 500 Vulcan twin (but that bike is only louder than electric motor bikes).

Making this bike cool is still a few steps away...
 
Ok, brought it home from my friend's garage.
Starts easy enough, runs steady. In fact it's a blast to ride, although I'm a bit tall at 6'1" ish.

Didn't tackle electric yet because while it looks messy, spark is good and headlight/brakelight work. Turn signals sometimes...

Next issue to address:
-bike dislikes shifting from 2 to neutral.
It shifts fairly predictably, accept from second the level just pushes down with no click or resistance and stays in second. I typically have to try a few times and ease the clutch out see if I've successfully found it, sometimes a few more tries. From neutral to first is easy, then up to second is easy.

-possibly related, the bike doesn't seem to snap in to gear when clutch is released.
It seems to ease into gear, like an automatic transmission. Is the engine oil the transmission oil as well? I think I used a heavier weight when I just changed the oil (being in Southern California I figured that was safe), could this lead to a slow trans connection?

I attempted to adjust the clutch lever (was the easiest thing I could get a wrench on, but it didn't seem to help anything, still tweaking it one way then the other)

Any ideas?
 
does sound possibly clutch related, especially the slow take up on releasing the clutch, it sounds like maybe the thicker oil may be slowing the meshing of friction and drive plates,

question - did you use car oil or just heavier bike oil, car oil will have 'friction modifiers' in it which will prevent the clutch from grabbing properly and will eventually stop it working altogether

question - have you lubed the clutch cable, if not give it a try, it could be the cable is not releasing properly and causing the gears to feel like they're taking up slowly
 
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